1. Field of the Invention
The present invention relates generally to electronic ballasts for lighting circuits, and relates more particularly to a digital electronic ballast with programmable features.
2. Description of Related Art
Electronic ballasts for fluorescent lighting applications are widely available and well known, particularly those that operate with a switching half-bridge. Such an electronic ballast is illustrated in U.S. Pat. No. 6,008,593 to International Rectifier Corporation. Electronic ballast controls have evolved to include a wide range of functionality and features, including power factor correction, fault detection and a variety of safety features.
A typical electronic ballast with a switching half-bridge provides an oscillator that is used to derive the switching signals for the half-bridge switches to appropriately direct current to various components at particular times to establish a power flow to the lamp load. One type of implementation of an electronic ballast using an oscillator involves connecting a Voltage Controlled Oscillator (VCO) with a feedback signal from the switching half-bridge to modify the VCO frequency in accordance with desired operational parameters. For example, in the case of fluorescent lamp dimming applications, the switching frequency of the electronic ballast can be adjusted to obtain particular dimming settings.
The use of a VCO in an electronic ballast entails a number of design challenges that include appropriately providing the input to the VCO to obtain the desired oscillation frequency. Typically, the input to the VCO is an analog signal, the magnitude of which determines the oscillator frequency. It is often the case that analog signals in electronic ballasts are subject to noise or interference, potentially causing the frequency of the VCO to vary unexpectedly.
Although the use of a VCO in an electronic ballast can reduce the requirements for external extensions to the electronic ballast control, a number of other desirable features typically use external components to program parameters for the features. For example, pre-heat time and frequency are often programmed by passive components connected externally to the ballast control. Similarly, an ignition frequency and maximum current level is programmed using external passive components. The operational switching frequency and other run time parameters determined in conjunction with the operation of the VCO are often set by external components that are specifically selected for a desired operational set point. For example, in the case of pre-heat time, an external capacitor may be connected to a control IC for the electronic ballast, so that pre-heat operation occurs while the capacitor charges, and terminates when the capacitor charges to a specified value. As a consequence of providing a number of features in the electronic ballast control, the number of external components are connected to the control to implement parameter selection for the features of the specific application.
One technique to implement a high degree of functionality without numerous external components in an electronic ballast control is to provide a micro-controller that typically includes a processor, memory, input and output control and optionally some form of a communication interface. By using a micro-controller as a basis of an electronic ballast control, a number of features can be realized including frequency control, pre-heat control, ignition control and feedback and a number of fault protections. However, micro-controller devices are relatively expensive with respect to electronic ballast controls, and can occupy a significant portion of the cost for providing an electronic ballast. In addition, the functionality of a micro-controller is often underused, because the majority of functions useful in providing a ballast control can be implemented with a fairly small percentage of the resources available in the micro-controller.
A difficulty often encountered in semiconductor circuits and in ballast controllers is a certain degree of inconsistency in performance parameters of the silicon circuits. For example, manufacturing tolerances and the nature of silicon circuit structures can combined to provide somewhat inconsistent results for operating silicon circuits at a desired set point. That is, to achieve a desired set point of operation for a silicon circuit, the circuit operational parameters should be obtained relative to the individual circuit, rather than as an independent static set point parameter. For example, an oscillator implemented in silicon designed to oscillate at a certain frequency may have a large degree of variation based on tolerances of circuit components and manufacturing tolerances. If a precise frequency is desired, an oscillator control should be provided that compensates for the individual tolerances of the oscillator circuit to provide a specific frequency.
Accordingly, it would be desirable to provide a simplified low cost ballast control that can be easily implemented with a simplified, programmable relative control that decreases uncertainties with respect to static circuit parameters.